The invention generally relates to the field of substrates or structures for producing electronic components, and to methods of producing such structures. It is applicable to semiconductor structures, in particular silicon on insulator (SOI) type structures. The invention also pertains to techniques for assembling layers or substrates on a substrate of a material having atypical surface properties.
FIG. 1 illustrates a conventional structure that includes a silicon layer 4 in which the components are located, and beneath which is a buried oxide layer 2. A silicon substrate 6 acts as a mechanical support. The oxide layer 2 provides insulation from stray currents and charges from ionized particles. It also provides good insulation between neighboring components formed in the same silicon layer, and in particular the oxide layer substantially reduces stray capacitance between the neighboring components.
The superficial silicon layer 4 is typically about 10 nanometers (nm) to 1000 nm thick, while the oxide layer 2 is on the order of several hundred nanometers thick, for example 400 nm. This type of structure can be obtained by using a “SIMOX” type process or by using a “wafer bonding” technique that relies on molecular bonding.
After producing an assembly such as that shown in FIG. 1, electronic components can be fabricated in the superficial silicon layer 4. The silicon layer 4 is therefore an active layer; the oxide layer 2 is an insulation layer; and the substrate 6 acts as a mechanical support that allows for processing of the assembly.
Proper operation of a component fabricated in the layer 4 depends upon various parameters. One of the parameters is heating, which can substantially limit the performance of the component. Thus, it would be advantageous to have a semiconductor on insulator type structure, and a method of producing such a structure, in which heating does not limit the performance of the subsequently produced components, or minimally limits the performance in comparison to known structures.
Furthermore, known techniques for assembling materials do not allow bonding of substrates or layers having an atypical surface that has a roughness of more than a certain limiting value on the order of 0.5 nm root mean square (rms), or which are difficult to polish, or which have a chemical composition that is not conducive to molecular bonding. Occasionally a need arises to bond materials that have a roughness above the limiting value, or wherein at least one of the materials is difficult to polish or has a chemical composition that is not conducive to bonding by molecular bonding, or to bond such a material with a layer or substrate of a material that may itself be compatible with direct bonding or molecular bonding.
Presented is a semiconductor structure that includes a substrate having a surface and being made of a material that provides atypical surface properties to the surface, a bonding layer on the surface of the substrate, and a further layer molecularly bonded to the bonding layer. The atypical surface properties preferably is at least one of a roughness of more than 0.5 nm rms, or a roughness of at least 0.4 nm rms that is difficult to polish, or a chemical composition that is incompatible with molecular bonding.
In an advantageous implementation, the substrate has a thermal conductivity of more than 1 W/cm/K, and may be made of diamond or aluminum nitride material. An intermediate layer to provide the surface layer having the atypical properties layer may be provided before forming the bonding layer. The intermediate layer preferably has a thermal conductivity coefficient that is higher than that of the substrate or that is between that of the bonding layer and that of the substrate. The intermediate layer may be made of silicon nitride.
In another advantageous variation of the invention, the further layer is a semiconductor material of at least one of silicon, germanium, gallium arsenide, silicon-germanium, a semiconductor Group III-Group V material, or a semiconductor Group II-Group VI material. The structure may form a Semiconductor-On-Insulator (SOI) structure. At least one portion of a surface of the bonding layer facing the further layer is a distance of 10 nm or less from the surface of a peak of the substrate to optimize heat transfer from the further layer to the substrate. The structure may also include at least one power component, or radio frequency (RF) component, or insulated gate bipolar transistor (IGBT) component, or metal oxide on silicon field effect transistor (MOSFET) component in the further layer of semiconductor material.
In yet another implementation, the further layer is a second substrate of a material having atypical surface properties. The atypical surface properties of the second substrate may include at least one of a roughness of more than 0.5 nm rms, or a roughness of at least 0.4 nm rms that is difficult to polish, or a chemical composition that is incompatible with molecular bonding.
A further aspect of the invention pertains to a method for fabricating such a semiconductor structure. The technique includes providing a substrate having a surface and being made of a material that provides atypical surface properties to the surface, providing a bonding layer on the surface of the substrate, smoothing the bonding layer to provide a surface that is capable of molecular bonding, and molecularly bonding a further layer to the bonding layer to form the structure. Again, the atypical surface properties preferably comprise at least one of a roughness of more than 0.5 nm rms, or a roughness of at least 0.4 nm nns that is difficult to polish, or a chemical composition that is incompatible with molecular bonding.
In an advantageous implementation, the method includes forming an intermediate layer on the substrate to provide the surface layer having the atypical properties before providing the bonding layer, the intermediate layer having a thermal conductivity coefficient that is higher than that of the substrate or that is between that of the bonding layer and that of the substrate. The intermediate layer may be composed of silicon nitride.
In an implementation, the further layer is made of a semiconductor material and at least one of a power component and a radio frequency (RF) component are fabricated in the further layer.
In another advantageous embodiment, the further layer is a second substrate of a material having atypical surface properties. The method may further include providing a second bonding layer on the second substrate before molecularly bonding, and the second substrate may be made of at least one of diamond or aluminum nitride.